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Extension Mimeo FNR, no. 069 (1977)

Page 001

FNR-69 April, 1977
forestry & natural resources
WILDLIFE AND FISHERIES
Fish Kills in Indiana—Their Causes and Prevention
by Herbert C. Krauch, Extension Specialist in Wildlife
For anyone who has a stocked pond—and there are thousands in Indiana—fish kills can be quite a problem. The condition called “fish kill” occurs when a number of fish in a given body of water die from a specific cause. Among these causes are chemical pollution, water quality (pH, hardness, oxygen supply, etc.), old age, physical disturbance, oxygen depletion, parasites and/or disease.
Often, fish kills can be anticipated and measures taken to prevent them.
The most common cause of fish kills in Indiana ponds is oxygen depletion. Oxygen depletion results when conditions create a greater demand for oxygen than the aquatic environment can produce. Oxygen depletion can occur in both winter and summer.
Winter kill
The conditions which produce oxygen depletion in winter are quite different from those which result in summer kills. The results of a winter kill are seldom noticed until spring when the ice melts. Then the dead fish, often the larger ones, are seen washing up along the edge. The large fish die first, because they require more oxygen, and will suffocate first.
During winter, most ponds are covered by ice. The oxygen supply under the ice depends on the passage of light through the ice and the resulting production of oxygen by the tiny algae in the water. If the ice is snow-covered, sunlight cannot penetrate and the plants cannot produce oxygen. Just an inch of snow can shut out as much as 90 per cent of the light; and five inches will shut out about 99 per cent.
The existing oxygen is then gradually used up by the respiration of the fish and by natural decay processes. If the snow remains on the ice long enough, the fish will suffocate.
Generally, not all the fish are killed. Some are more resistant to low oxygen levels than others. In addition, poor conditions may not exist uniformly throughout the pond. So, the result may be that the larger fish, such as bass, die, leaving the smaller,
immature fish in the pond. This creates conditions further unbalancing the pond population very quickly.
Preventing winter kill
Pond design is important to preventing winter kill. In Indiana, ponds should be at least eight feet deep over at least 25 per cent of the pond area. Greater depths are even better.
If, however, an eight foot depth isn’t possible, a six foot depth over at least half the pond is an alternative. Deepening the pond and/or removing build-ups of dead and decaying organic matter from the pond will help preserve the oxygen.
Removing snow cover from at least 50 per cent of the pond surface may help. But, unless the ice is thick enough to hold a person or small tractor, this can be very dangerous. Be sure the ice is sufficiently thick! Chopping holes in the ice won’t help.
Summer kill
The most common cause of summer fish die-offs is the same as winter kill—oxygen depletion. But the causes of depletion may be more complex than with winter kills. Summer depletion can result from oxygen removal by excessive quantities of plants, animals or decaying organic matter.
This can be caused by overstocking, overfeeding, over fertilization, pollution from barns, feedlots, improper septic drainage, or chemical treatment of aquatic weeds during the critical months—June through September.
Another common cause of summer fish kills occurs when a dense growth of submerged aquatic plants or algae in a pond dies suddenly from natural causes or from herbicides.
The decay process from the dead plants may use up the oxygen in the water. This type of summer fish kill almost always happens about sunrise, when the dissolved oxygen is at its low point for the day.
PURDUE UNIVERSITY • COOPERATIVE EXTENSION SERVICE • WEST LAFAYETTE, INDIANA 47907

FNR-69 April, 1977
forestry & natural resources
WILDLIFE AND FISHERIES
Fish Kills in Indiana—Their Causes and Prevention
by Herbert C. Krauch, Extension Specialist in Wildlife
For anyone who has a stocked pond—and there are thousands in Indiana—fish kills can be quite a problem. The condition called “fish kill” occurs when a number of fish in a given body of water die from a specific cause. Among these causes are chemical pollution, water quality (pH, hardness, oxygen supply, etc.), old age, physical disturbance, oxygen depletion, parasites and/or disease.
Often, fish kills can be anticipated and measures taken to prevent them.
The most common cause of fish kills in Indiana ponds is oxygen depletion. Oxygen depletion results when conditions create a greater demand for oxygen than the aquatic environment can produce. Oxygen depletion can occur in both winter and summer.
Winter kill
The conditions which produce oxygen depletion in winter are quite different from those which result in summer kills. The results of a winter kill are seldom noticed until spring when the ice melts. Then the dead fish, often the larger ones, are seen washing up along the edge. The large fish die first, because they require more oxygen, and will suffocate first.
During winter, most ponds are covered by ice. The oxygen supply under the ice depends on the passage of light through the ice and the resulting production of oxygen by the tiny algae in the water. If the ice is snow-covered, sunlight cannot penetrate and the plants cannot produce oxygen. Just an inch of snow can shut out as much as 90 per cent of the light; and five inches will shut out about 99 per cent.
The existing oxygen is then gradually used up by the respiration of the fish and by natural decay processes. If the snow remains on the ice long enough, the fish will suffocate.
Generally, not all the fish are killed. Some are more resistant to low oxygen levels than others. In addition, poor conditions may not exist uniformly throughout the pond. So, the result may be that the larger fish, such as bass, die, leaving the smaller,
immature fish in the pond. This creates conditions further unbalancing the pond population very quickly.
Preventing winter kill
Pond design is important to preventing winter kill. In Indiana, ponds should be at least eight feet deep over at least 25 per cent of the pond area. Greater depths are even better.
If, however, an eight foot depth isn’t possible, a six foot depth over at least half the pond is an alternative. Deepening the pond and/or removing build-ups of dead and decaying organic matter from the pond will help preserve the oxygen.
Removing snow cover from at least 50 per cent of the pond surface may help. But, unless the ice is thick enough to hold a person or small tractor, this can be very dangerous. Be sure the ice is sufficiently thick! Chopping holes in the ice won’t help.
Summer kill
The most common cause of summer fish die-offs is the same as winter kill—oxygen depletion. But the causes of depletion may be more complex than with winter kills. Summer depletion can result from oxygen removal by excessive quantities of plants, animals or decaying organic matter.
This can be caused by overstocking, overfeeding, over fertilization, pollution from barns, feedlots, improper septic drainage, or chemical treatment of aquatic weeds during the critical months—June through September.
Another common cause of summer fish kills occurs when a dense growth of submerged aquatic plants or algae in a pond dies suddenly from natural causes or from herbicides.
The decay process from the dead plants may use up the oxygen in the water. This type of summer fish kill almost always happens about sunrise, when the dissolved oxygen is at its low point for the day.
PURDUE UNIVERSITY • COOPERATIVE EXTENSION SERVICE • WEST LAFAYETTE, INDIANA 47907